Gagnon was born March 7, 1925 in Manchester, New Hampshire, the only child of French Canadian immigrants from Disraeli, Quebec, Henri Gagnon and Irène Marcotte. He grew up without a father. His parents separated when he was an infant, though they never divorced. When he was old enough, he worked alongside his mother at a local shoe factory. He also worked as a bicycle messenger boy for the local Western Union.

On February 19, 1945, Gagnon landed on the southeast side of Iwo Jima with E Company, 2nd Battalion, 28th Marines on "Green Beach 1", which was the closest landing beach to Mount Suribachi on the southern end of the island. On February 23, Gagnon, a battalion runner (messenger) for Easy Company,[1] participated in what was most likely the most celebrated American flag raising in U.S. history.

On the morning of February 23, a 40-man combat patrol (mostly from Third Platoon, Easy Company) was sent to climb up Mount Suribach and siege and occupy the crest. First Lieutenant Harold Schrier, E Company's executive officer, who volunteered to lead the patrol, was to raise an American flag on top to signal that the mountaintop was captured. Once Schrier and his men were on top, a Japanese iron water pipe was found to attach the flag to. Then Lt. Schrier, Platoon Sgt. Ernest Thomas, and Sgt. Henry Hansen raised the flag, planting the flagstaff into the ground.[2] Seeing the raising of the national colors immediately caused loud cheers from the Marines, sailors, and Coast Guardsmen on the south end beaches of Iwo Jima and from the men on the ships near the beach.

Gagnon is included in a portion of the color film of the flag-raising taken by Sgt. Bill Genaust, USMC, excerpted from the 1945 "Carriers Hit Tokyo" newsreel.

Some two hours after the first flag was raised on Mount Suribachi, the Marines decided that a larger flag should be flown on Mount Suribachi in order for the American flag to be seen more easily from the ships, beaches, and land off and around the north side of Mount Suribachi where most of the Japanese soldiers were located and heavy fighting would occur in the days ahead. Sgt. Michael Strank a squad leader of Second Platoon, E Company, was ordered to take three Marines from his squad up Mount Suribachi to raise the replacement flag. He selected Cpl. Harlon Block, Pfc. Ira Hayes, and Pfc. Franklin Sousley. Gagnon, a Second Battalion, 28th Marines runner (messenger) for E Company, was ordered to take the replacement flag up the mountain and return with the first flag that was flying on top.

Once Gagnon and the four Marines were on top, a Japanese pipe was found by Hayes and Sousley and taken near the first flag position, After the replacement flag was attached to it, Lt. Schrier ordered the replacement flag/flagstaff to be raised and the first flag/flagstaff lowered at the same time. Sgt. Strank, Cpl. Block, Pfc. Hayes, Pfc. Sousley, Pfc. Gagnon (called by Strank to help), and Pfc. Harold Schultz raised the replacement flag (Navy corpsman John Bradley was incorrectly identified as a flag-raisers until June 23, 2016).[3] In order to keep the flagstaff in a vertical position, rocks were added around the base and a rope was tied to it and staked down in three spots. Associated Press combat photographer Joe Rosenthal took a black and white photograph of this flag raising which became world famous. The flag raising was also filmed in color by Sgt. Bill Genaust who was killed in action a few days later.

A diagram identifying the six men who raised the replacement flag on Mount Suribachi

On March 14, 1945, a third American flag was officially raised by two Marines at Kitano Point at the northern end of the island. The flag that Gagnon helped raise in the two unofficial flag-raisings on February 23, was lowered and taken to Marine headquarters. On March 27, Gagnon and the 2nd Battalion, 28th Marines left Iwo Jima and both flags that were flown on Mount Suribachi were taken to Marine Headquarters in Washington, D.C. after the 2nd Battalion, 28th Marines returned to Hawaii.

In 1991, former Marine Lt. George Greeley Wells, who was the 2nd battalion, 28th Marines, adjutant in charge of carrying the American flag(s) for the battalion, stated in The New York Times that he was ordered by the battalion commander on February 23, 1945 to get a large replacement flag for the top of Mount Suribachi, and that he (Wells) ordered Gagnon, his E Company runner, to get a flag from a ship on shore — possibly the USS Duval County (LST-758).[4] Wells stated that this flag was the one taken up Mount Suribachi by Gagnon to be given to Lt. Schrier of Company E, with a message for Schrier to raise this flag and return the other smaller flag raised earlier on Mount Suribachi back to Gagnon. Wells also stated, that he had handed the first flag to Schrier that Schrier took up Mount Suribachi, and when this flag was returned to him by Gagnon, he secured the flag until it was delivered to Marine Headquarters after the 2nd battalion returned to Hawaii from Iwo Jima.[5][6][7][8]

On March 27, 1945, when Gagnon and the 28th Marines left Iwo Jima for Hawaii, Gagnon was aboard the transport USS Winged Arrow (AP-170),[9]On March 30, President Franklin Roosevelt ordered the six flag rasiers in Rosenthal's flag raising photo to be sent to Washington, D.C.. Gagnon was the first to be identified, and as one of the three surviving flag raisers in the photo on April 7, followed by PhM2c. Bradley and Pfc. Hayes (Sgt. Strank, Cpl. Block, and Pfc. Franklin Soulsey were killed in action; Block was mistaken to be Sgt. Henry Hansen until a Marine investigation in 1946 revealed in 1947 it was Block). Once in Washington, D.C., the three were assigned to temporary duty with the Finance Division, United States Department of the Treasury, for appearances and participation in connection with the Seventh War Loan drive (bond selling tour) from May 11 through July 4 (Hayes left the bond tour on May 25 for E Company in Hawaii). The tour was kicked off with a flag raising at the Nation's capitol on May 9 and went through 33 U.S cities raising billions of desperately needed dollars to help boost morale and win the war.[10][11] The three flag-raisers had the actual flag they had raised on Mount Suribachi with them during the bond tour.

In July 1945, Gagnon was ordered to San Diego for further transfer overseas. He married Pauline Georgette Harnois, of Hooksett, New Hampshire, in Baltimore, Maryland, on July 7, 1945. By September, he was on his way overseas again, this time with the 80th Replacement Draft. On November 7, 1945, he arrived at Tsingtao, China, where he joined E Company, 2nd Battalion, 29th Marines, 6th Marine Division. He later served with the 3rd Battalion, 29th Marines. In March 1946, he had been on duty with the U.S. occupation forces in China for about five months before he boarded a ship at Tsingtao at the end of the month for San Diego. Gagnon arrived in San Diego on April 20, 1946. He was promoted to Corporal and was honorably discharged at Camp Pendleton, California, on April 27, 1946.

The Marine Corps War Memorial (also known as the Iwo Jima Memorial) in Arlington, Virginia which was inspired by Rosenthal's photograph of the second flag-raising on Mount Suribachi was dedicated on November 10, 1954.[12] Gagnon is depicted as the second bronze statue from the bottom of the flagstaff with the 32 foot (9.8 M) bronze statues of the other five flag-raisers on the monument (as of June 23, 2016, Franklin Sousley and Harold Schultz are depicted as the third and fifth bronze statues from the bottom of the flagstaff).[13]

PresidentDwight D. Eisenhower sat upfront with Vice PresidentRichard Nixon, Deputy Secretary of DefenseRobert Anderson, and General Lemuel C. Shepherd, the 20th Commandant of the Marine Corps, during the dedication ceremony. Two of the three surviving flag-raisers depicted on the monument, Ira Hayes and Gagnon, were seated together with John Bradley (he was incorrectly identified as a surviving flag-raiser)[14] in the front rows of seats along with relatives of the those who were killed in action on the island.[15] Speeches were given by Richard Nixon, Robert Anderson who dedicated the memorial, and General Shepherd who presented the memorial to the American people.[16] Inscribed on the memorial are the following words:

In Honor And Memory Of The Men of The United States Marine Corps Who Have Given Their Lives To Their Country Since 10 November, 1775

On February 19, 1965, while working as an airline sales representative for Delta Air Lines, Gagnon visited Mount Suribachi after visiting Tokyo with his wife and son.[17][18]

René Gagnon, Jr. commented in 2014 that his father René Gagnon, Sr. opened a travel agency and did accounting work and was not a bitter alcoholic, and in his last job, he had worked as head of maintenance at an apartment complex in Manchester where he suffered a heart attack in the boiler room.[19] According to the book Flags of Our Fathers (2000), in his latter years Gagnon only participated in events that were at his wife's urging, events praising the U.S. flag raising on Iwo Jima. She enjoyed the limelight, whereas he, by that time, no longer did.

“

At the age of 53, he bitterly inventoried his lost "connections" — the jobs promised him by government people when he had been at the height of his fame, jobs that never materialized. "I'm pretty well known in Manchester," he told a reporter. "When someone who doesn't know me is introduced to me, they say 'That was you in The Photograph?' What the hell are you doing working here? If I were you, I'd have a good job and lots of money.'"[20]

Gagnon died on October 12, 1979, at age 54, in Manchester, New Hampshire. He resided in Hooksett, New Hampshire, near Manchester, and left behind his wife Pauline Gagnon (January 16, 1926 – January 16, 2006) and son René Gagnon, Jr. He was buried at Mount Calvary Cemetery in Manchester. At the request of his widow, a government waiver was granted on April 16, 1981, and his remains were re-interred in Section 51, Grave 543 of Arlington National Cemetery on July 7. Inscribed on the back of his Arlington headstone are the words:

FOR GOD AND COUNTRY

HE RAISED OUR FLAG IN BATTLE
AND SHOWED A MEASURE OF HIS PRIDE
AT A PLACE CALLED IWO JIMA